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The Chevy Bolt — A Deeper Look

chevy_bolt_mules

The Bolt announcement has revealed significant details of the new Chevy Bolt electric vehicle. The pleasing combination of range, performance, and price are at a new level for electric vehicles.

With the current information, we can now compare to figures for the Sonic/Trax, the platform the Bolt shares, as well as the top-selling Nissan Leaf.

  • Trax = 167″L x 70″W x 65″H (Weight 3,048 lbs)
  • Bolt = 164″L x 69.5″W x 62.8″H (Weight 3,580 lbs)
  • Leaf = 175″L x 70″W x 61″H (Weight 3,256 lbs)

The Bolt is very short, even short compared to the Sonic/Trax, but is midway in height between the smaller Sonic and the mini SUV Trax, at almost 63 inches. The height and width make it have more frontal area than the Leaf.

The height adds to its interior volume. Cargo volume behind the rear seat is 16.9 cubic feet, though, a little less than the Leaf at 24 cubic feet. Passenger area specifications are fairly equal, with a bit more leg room in the rear for the Bolt. Keeping it short helps to keep the weight down, but it will be shown that the height and shortness make aerodynamic drag more difficult to manage.

Charging

The charging options are familiar, with slightly higher-power onboard charging at 7.2 kW compared to standard 6.6 kW, but that only results in about 25 miles per hour of charging. The DC fast charger is faster, but it takes 30 minutes for only 90 miles of charging, compared to Tesla’s 170 miles of charge in 30 minutes.

The Battery Pack

The 60 kWh battery pack, at 960 lbs (435 kg), has excellent specific energy, a bit less than Tesla’s 1200 lbs, 85 kWh pack.

Chevy Bolt battery

The specific energy numbers are:

  • Bolt 0.138 kWh/kg
  • Model S 0.156 kWh/kg

The Bolt pack is described as having 96 cell groups with 3 cells per group. At an average voltage per cell of 3.8 volt (V), the pack voltage consisting of 96 cells in series is about 365V. With a pack energy capacity of 60 kWh, and three cells in parallel per group, each cell has a rating of about 55 amp-hour. From the description of high nickel and manganese content and low thermal sensitivity, it is likely that this is a variation of the nickel-manganese-cobalt-oxide chemistry, NMC, an excellent choice for electric vehicle applications.

The cells are pouch type and fit neatly in a space-saving skateboard arrangement under the floor with a total 5 sections of 10 modules. At the back, the modules are stacked 2 high. This arrangement adds to the internal roominess and low center of gravity of the vehicle. The pack is also liquid cooled, which is likely to aid in long life, reliability, and improved performance over a wide range of conditions.

Range, Range, Range

GM won’t release range specifications until the EPA tests the vehicle, but says 200 miles of range. That appears likely for city driving. With the same 60 kWh battery capacity and a curb weight of only 3,580 lbs, compared to the Model S 60 at 4,407 lbs,
the Bolt should easily be able to travel more than 208 miles from full charge to full discharge in the city. Range at highway speeds is more important, because highway driving often matches the longest trips. For average speeds and travel distances between rest stops, 3-hour driving time for 200 miles of range means average speeds of 65 to 70 mph. The highway range must be measured by those standards.

At those speeds, aerodynamic drag dominates.

battery Wh:mile

Whpermilevsspeed

Aerodynamics

To determine aerodynamic drag, we have to look at the effective frontal area, the product of coefficient of aerodynamic drag, Cd, and frontal area. For comparison, the Model S is 77.3 inches wide by 56.5 inches tall, with mirrors folded.

The Bolt is 69.5 inches wide by 62.8 inches tall.

Frontal Area

  • Model S — 4,367 square inches
  • Bolt — 4,365 square inches

The two cars have virtually identical frontal area. The Model S has a Cd of 0.24. The Bolt designers did their best to lower Cd; however, it’s unlikely they achieved such a low Cd. To see why, we need to look at airflow. The goal of aerodynamics is to achieve the lowest drag. The subject is deep, fraught with details, and full of pitfalls for the educated, experienced, and neophyte alike. However, there are some principles that apply.

A member of the winning team for the tandem category in a competition once told me these words, “move the air only once, and put it back gently.” In aerodynamic terms, no sharp increases or decreases in pressure. The goal is “laminar flow,” the smooth flow of air around the object, without turbulence. The ideal shape is a teardrop, with a rounded front, and tapered rear. The Bolt shape has a smooth front with no sudden changes, good for aerodynamics, and a tail described as a kammback, shaped like a bullet. There is some tapering, but the abrupt rear end stops before the shape has reduced a lot. Some effort has been made to improve it with the rear spoiler extending the roof further to the rear. Below is a video of how this behaves in a wind tunnel. The upper and lower airflows are shown in two different colors.

If you look carefully, you can see two whorls or vortices at the rear that rotate in opposite directions, one on top, and one on the bottom.

Now, let’s take a look at another design, the boat tail. The boat tail slices off the rear of the teardrop shape farther toward the rear than the kammback.

Notice how much smaller the vortices are and how the upper vortex is almost eliminated. As a rule of thumb, the larger the vortices and the more upper and lower vortices, the higher the drag. The boat tail design results in a lower drag coefficient. A glance at the Model S reveals that it has a boat tail appearance. The kammback of the Bolt is likely to result in higher Cd and drag.

The Leaf, with a similar kammback, achieves a Cd of 0.28. There are many other details to aerodynamics, but the overall shape is a good indicator of performance.

The Model S range vs speed graph shows the range is just around the 200 mile mark at the speeds of interest. From those facts, it looks like the Bolt will have just under 200 mile range at the highway speeds of interest. The large frontal area and high Cd of the Bolt mean that the same size battery pack as the Model S 60, will not carry it as far at highway speeds. If, instead, the frontal area was lower and the Cd was also, the size of the battery pack could be decreased or the range increased.

It’s no secret that GM wants this early introduction to compete with the upcoming Tesla Model 3 in March. It might be speculated that a Model 3 with a 20% reduction in frontal area, and with a similar sedan shape and Cd, might result in at least a 20% lighter car with 20% less frontal area. If that is the case, the battery might also be 20% smaller, or about 50 kWh, resulting in lower cost. Coincidentally, a 20% lighter car than the Model S would be 3,520 lbs.

Performance

The claimed figure of 7 seconds 0–60 mph matches the battery and motor capability. The motor, battery, and controller determine the acceleration. With the large battery pack and about 165 amp-hours at about 365V, there is ample power available from the battery. An NMC battery should be easily capable of 3C performance, leading to power ratings of 200 horsepower, consistent with the motor rating. Doubtless, the designers decided to take advantage of this capability. That kind of performance will win many converts, if they don’t mind the package being more family friendly. The heavily tapered front end does impart a sporty flavor.

What’s Not to Like?

This is the first affordable, 200-mile range, electric vehicle on the market. That makes it a unique item of value. As the first of its kind, with a price in the mid $30,000s, good interior space, zippy acceleration, and long range, it promises to be a popular item. It’s hitting a lot of wants and needs. It could be used for medium trips up to ~200 miles under good conditions. Even with slightly higher aerodynamic drag, the large battery makes for long range. Although it does not charge as fast as a Model S, only 90 miles in a half hour, about a 1 hour stop every 3 hours (~200 miles), it could still extend travel relatively long distances in decent time. While not as pleasing as the faster-charging and longer-range Model S, it does have a lower initial entry price. The biggest difficulty now is the lack of an extensive SAE DC fast-charger cross-country network. That is likely to change with the introduction of the Bolt, spurring third-party chargers to expand.

 
 
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Written By

has studied wind, electric vehicles, and environmental issues. An electrical engineer familiar with power and electronics, he has participated in the Automotive X Prize contest. He is an avid writer, specializing in electric vehicles, batteries, and wind energy.

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